The invention relates to an installation for the treatment of municipal and/or industrial wastewater, comprising a basin that receives the water to be treated and a series of disks with packing, said disks being parallel, vertical mounted on a horizontal shaft, partially immersed in the water to be treated and rotated in such a way that the biomass that grows on the surface of the packing of the disks is alternately brought into contact with the water to be treated and with oxygen from the air by a trickling effect.
Such an installation is particularly suitable for treating the wastewater from towns or communities having 2000 equivalent inhabitants (EI) or fewer.
Taking the example of France which has over 36 000 towns, there are about 34 650 towns with fewer than 2000 inhabitants, which corresponds to a population of about 25 000 000 inhabitants, i.e. about 40% of the total population of France.
The national stock of purification plants in France is made up of about 15 000 purification plants, about 60% of which smaller in size than 2000 EI. Close to 50% of these small installations are over 15 years old, while 30% are over 20 years old. It may therefore be considered that this old stock will be renewed in the short term.
Biological treatment by activated sludge has often been the treatment of choice, although this technique is not suited to such small installations because of the high investment costs and above all a high operating cost. The efficiency of activated-sludge installations is often mediocre for capacities of less than 2000 EI.
The alternatives to treatment by activated sludge for small installations are mainly treatment systems employing bacterial beds, natural lagoons, filters planted with rushes, biological disks.
The biological disk technology is widely used throughout the world, especially in northern Europe, and is perfectly suited to the smallest installations ranging in size from a few tens to a few thousand equivalent inhabitants. The compactness and low height of biological disk installations are major advantages for integrating them discretely into exacting sites. It is even possible to integrate these installations into building basements or into small separate buildings. Their low electrical consumption is noteworthy.
In particular in France, the objectives and constraints of the new Decree of Jun. 22, 2007 “relatif à la collecte, au transport et au traitement des eaux usées des agglomérations d'assainissement ainsi qu'à la surveillance de leur fonctionnement et de leur efficacité, et aux dispositifs d'assainissement non collectif recevant une charge brute organique supérieure à 1.2 kg/j de DBO5 [relating to the collection, transportation and treatment of wastewater from sewage catchment areas and also to the monitoring of their operation and their efficiency, and to noncommunal treatment plants receiving raw water with a BOD5 organic content greater than 1.2 kg/d]” (20 equivalent inhabitants) published in the Journal Officiel [Official Journal] dated Jul. 14, 2007, have been met.
ParameterMax. concentrationMin. efficiencyBOD535 mg/l60%COD60%SM50%
It will be recalled that the expression BOD5 denotes the biochemical oxygen demand measured at the end of 5 days, COD denotes the chemical oxygen demand and SM denotes the suspended matter.
The treatment of wastewater using biological disk technology involves the process of attached growths. The purifying biomass grows on the rotating disks and their packing, especially a plastic packing with, a specific surface area close to 150 m2/m3. The amount of biomass that will grow on the support depends on the specific surface area of the contacting material and on the thickness of the biofilm, which can vary from a few hundred microns to several millimeters.
In the case of biological disks, the support for the attached growths can rotate about an axis. The rotation of the disks enables the biomass to be brought into contact with the pollution to be treated and to oxygenate the medium through the water trickling effect when the zone bearing the biomass leaves the liquid.
The linear speed of the disk relative to the water is higher toward the outside than at the center, and the trickling is thus greater toward the periphery of the disk, with better oxygenation and more effective erosion of the biofilm. In fact, it is found in treatment units that the biofilm can reach a thickness of several millimeters near the rotation axis, whereas this thickness is smaller to the outside.
The surface layer of the biofilm remains an aerobic zone, but the deep layers are in a state of anaerobiosis (significant black layers). This generally observed situation prevents the treatment from being optimized.
In addition, the area wetted per revolution is considerably greater to the outside than to the inside of the disks (about ten times greater for the example of a 2 m diameter).
Consequently, the further away from the center, the thinner the biofilm, the better the oxygenation and the higher the contact area. It is therefore advantageous to increase a developed surface area to the outside of the disks.
To approximate the notion of a load to be treated per quantity of biomass (volume load of activated sludge), it is general practice to refer to a surface load: gBOD5/d per m2 of developed surface area. The oxygenation capacity is low, which means that the charge to be treated is limited, especially on the first stage for disks arranged in series. Because of this low oxygenation capacity, it is not possible in general to use biological disk technology as first treatment step, before a complementary biological treatment. Furthermore, it is difficult to envisage this biological disk technology in tertiary treatment for just nitrogen treatment (nitrification) due to the impossibility of controlling the thickness of the biofilm, which necessarily has to be very thin for this specific treatment.
Thus, according to the prior art, treatment using biological disks is generally reserved for the main treatment, for the elimination of carbon-containing pollution, and, to a lesser extent, for nitrogen-containing pollution with limited nitrification performance.
EP-A-0 182 380 relates to an installation for the treatment of wastewater using a series of disks or drums with packing, which are parallel, mounted on a horizontal shaft, partially immersed in the water to be treated and rotated. According to one embodiment, the packing of the drums is formed by tubes comprising latices or networks. The abovementioned drawbacks remain in such an installation.